1. Field of the Invention
The present invention relates to a transmitter and receiver circuit using microwaves or millimetric-waves.
2. Background Art
In a transmitter and receiver circuit using microwaves or millimetric-waves, high-frequency signals are received from a plurality of antennas, and converted down to intermediate frequencies by a mixer (for example, refer to Japanese Patent Laid-Open No. 2001-7651).
FIG. 9 is a block diagram showing an example of a conventional transmitter and receiver circuit. The transmitter and receiver circuit has a plurality of antennas 11a to 11e that receive high-frequency signals, a switch 41 that selects any one of the plurality of antennas 11a to 11e, a low-noise amplifier 42 that amplifies high-frequency signals from the selected one of the antennas 11a to 11e, a local oscillator 12 that generates local oscillation signals, a power amplifier 13 that amplifies the local oscillation signals, a coupler 14 that divides the amplified local oscillation signals, and a mixer 15 that converts the frequency of the high-frequency signals from the selected one of the antennas 11a to 11e in response to the local oscillation signals inputted from the local oscillator 12.
If the switch 41 is provided in a part of a high-frequency circuit, since the passage loss of the switch 41 cannot be ignored in micro wave or millimetric-wave band, the noise index of the transmitter and receiver circuit is elevated. Whereas, although the loss of the switch 41 can be compensated by installing a low-noise amplifier 42, the number of components is increased, and the manufacturing costs of the transmitter and receiver circuit are elevated.
FIG. 10 is a block diagram showing another example of a conventional transmitter and receiver circuit. In this transmitter and receiver circuit, a plurality of mixers 15a to 15e connected to a plurality of antennas 11a to 11e, respectively, operate in parallel. By installing the switch in the local oscillation circuit side instead of installing the switch in the high-frequency circuit side, receiving channels can be switched without increasing the receiving noise. However, in this transmitter and receiver circuit, a five-way distributor 43 corresponding to the number of mixers 15a to 15e must be used.
Since the theoretical loss of the five-way distributor 43 is as large as 7 db, to excite the mixers 15a to 15e, amplifiers 44a to 44e having a gain of 7 db or more are required corresponding to the number of mixers 15a to 15e. However, if amplifiers 44a to 44e are installed, the size of the transmitter and receiver circuit is enlarged, and manufacturing costs are elevated. Especially in the millimetric-wave band, the design of a distributor having a low loss and equipartition characteristics is extremely difficult.